1. Field of the Invention
The present invention relates to a green-emitting phosphor composition and a cathode ray tube (CRT) manufactured using the same, and more particularly, to a green-emitting phosphor composition having improved luminance characteristics, degradation characteristics and afterglow characteristics, and a CRT adopting the same.
2. Description of the Related Art
Recently, with the advent of HDTV (high definition television) broadcasting, demands for projection type CRTs which can attain 40-inch or larger screens have been increasing for both home and business purposes. In the case of a projection type CRT, the fluorescent screen of a projection type CRT requires a brightness of several to several tens of times that of a direct viewing type CRT because an image is projected onto a 40 inch, 50 inch or larger screen. Thus, the fluorescent screen of a projection type CRT should be excited under the condition in which a high-density current of several to several tens of times that of a direct viewing type CRT is applied. However, this results in deterioration of luminance, which is particularly serious in green-emitting phosphors which contribute to about 70% of the luminance.
As components of the conventional green-emitting phosphor, Y3Al5O12:Tb, Y3(Al, Ga)5O12:Tb, LaOCl:Tb, Zn2SiO4:Mn, InBO3:Tb or Y2SiO5:Tb, which have improved characteristics compared to Y2O2S:Tb, Gd2O2S:Tb or ZnS:Cu, Al, are used alone. However, these single-substance green-emitting phosphors do not fulfil all characteristics required for a projection type CRT. Thus, mixed green-emitting phosphors obtained by mixing Zn2SiO4:Mn or InBO3:Tb with Y3(Al, Ga)5O12:Tb, Y2SiO5:Tb or pigment-attached Y3(Al, Ga)5O12:Tb in an appropriate mixture ratio are put into practical use.
For example, Japanese Patent Laid-Open Publication No. hei 4-161483 describes a mixed green-emitting phosphor obtained by mixing Y3(Al, Ga)5O12:Tb with Zn2SiO4:Mn in order to improve color purity. U.S. Pat. No. 4,559,469 describes a mixed green-emitting phosphor obtained by mixing Zn2SiO5:Mn with Y2SiO5:Tb in order to improve color purity.
However, the Zn2SiO4:Mn phosphor used for improving color purity has poor degradation, luminance and afterglow characteristics under the condition in which a high-density current is applied, while exhibiting excellent color purity. Thus, the Zn2SiO4:Mn phosphor mixed for improving color purity considerably deteriorates luminance and degradation characteristics of the mixed green-emitting phosphor.
To solve the above problems, it is an object of the present invention to provide a new green-emitting phosphor composition which can replace a conventional green-emitting phosphor composition containing a Zn2SiO4:Mn phosphor.
It is another object of the present invention to provide a cathode ray tube (CRT) manufacturing method using the new green-emitting phosphor composition.
Accordingly, to achieve the first object, there is provided a green-emitting phosphor composition including 5 to 50 wt % of a SrGa2S4:Eu phosphor and 50 to 95 wt % of a Y2(Al, Ga)5O12:Tb phosphor.
According to another aspect of the present invention, there is provided a green-emitting phosphor composition including 5 to 20 wt % of a SrGa2S4:Eu phosphor, 60 to 90 wt % of a Y2(Al, Ga)5O12:Tb phosphor and 5 to 20 wt % of a LaOCl:Tb phosphor.
To achieve the second object, there is provided a cathode ray tube manufactured using the green-emitting phosphor composition.
A mixed green-emitting phosphor composition comprising a mixture of a SrGa2S4:Eu phosphor and a Y3(Al, Ga)5O12:Tb phosphor or a mixture of a SrGa2S4:Eu phosphor, a Y3(Al, Ga)5O12:Tb phosphor and a LaOCl:Tb phosphor according to the present invention has excellent luminance, afterglow and degradation characteristics under the condition in which a high-density current is applied, so that it is capable of replacing a conventional green-emitting phosphor composition comprising a mixture of a Zn2SiO4:Mn phosphor, Y3(Al, Ga)5O12:Tb phosphor and/or a LaOCl:Tb phosphor.
That is to say, since the Y3(Al, Ga)5O12:Tb phosphor has poor color purity while exhibiting excellent luminance saturation characteristic and excellent degradation characteristic under high-voltage and high-current conditions, it is used in a mixture with a Zn2SiO4:Mn phosphor having good color purity. However, the Zn2SiO5:Mn phosphor has a very long afterglow time, that is, about 30 ms, and is poorest in view of luminance and degradation characteristics among single-substance phosphors for a projection type CRT. Therefore, the green-emitting phosphor compositions according to the present invention have improved luminance, degradation and afterglow characteristics by replacing a Zn2SiO4:Mn phosphor contained in the conventional green-emitting phosphor composition comprising the Zn2SiO4:Mn phosphor, a Y3(Al, Ga)5O12:Tb phosphor and/or a LaOCl:Tb phosphor, with a SrGa2S4:Eu phosphor having excellent luminance and degradation characteristics and a short afterglow time, that is, about 300 xcexcs.
First, a method of manufacturing a SrGa2S4:Eu phosphor of the present invention will be described in detail.
0.5 to 1.5 mol of a strontium salt such as strontium carbonate or strontium sulfate, 1.5 to 2.5 mol of a gallium salt such as gallium oxide or gallium sulfate and 0.001 to 0.05 mol of europium oxide are mixed and then an appropriate amount of a solvent such as sodium bromide, ammonium chloride or sodium chloride is mixed therewith. Subsequently, the mixture is put into an alumina vessel and then fired at 700 to 900xc2x0 C. under a hydrogen sulfide gas atmosphere for 1 to 4 hours. Then, the resultant material is cooled, washed, dried and sieved to obtain a SrGa2S4:Eu phosphor.
After screen layers are manufactured using a SrGa2S4:Tb phosphor prepared in the above-described manner and prior art Y3(Al, Ga)5O12:Tb phosphor, a LaOCl:Tb phosphor and a Zn2SiO4:Mn phosphor by sedimentation, the emission characteristics are evaluated using a demountable system BM-7 and the evaluation result is described in the following Table 1. The evaluation conditions are an acceleration voltage of 20 kV, a driving current Ik of 60 xcexcA, a raster size of 2.0 cmxc3x972.0 cm and a degradation evaluation time of 600 seconds.
In Table 1, Luminance1) represents a relative luminance when the luminance of the Y3(Al, Ga)5O12:Tb phosphor is set to 100%.
Referring to Table 1, under high voltage and high current conditions, the Y3(Al, Ga)5O12:Tb phosphor exhibits an excellent luminance characteristic, specifically an excellent degradation characteristic, while exhibiting considerably poor color purity. Thus, the Y3(Al, Ga)5O12:Tb phosphor has been conventionally used in a mixture with a Zn2SiO4:Mn phosphor having excellent color purity. However, the Zn2SiO4:Mn phosphor is very poor in view of luminance and degradation characteristics. On the other hand, a SrGa2S4:Eu phosphor is excellent in view of color purity and luminance characteristics and is relatively superior in view of degradation characteristics than the Zn2SiO4:Mn phosphor, and thus a mixed green-emitting phosphor composition thereof exhibits excellent luminosity characteristics. However, the degradation characteristics of the SrGa2S4:Eu phosphor are lower than those of the Y3(Al, Ga)4O12:Tb phosphor. Thus, the present invention provides a mixed phosphor composition of 5 to 50% by weight of a SrGa2S4:Eu phosphor and 50 to 95% by weight of a Y3(Al, Ga)4O12:Tb phosphor, for suppressing degradation characteristic of the phosphor composition from being greatly lowered. Here, the contents of the SrGa2S4:Eu phosphor and the Y3(Al, Ga)4O12:Tb phosphor are preferably 7 to 20% by weight and 80 to 93% by weight, respectively.
Also, the present invention provides a phosphor composition further including a LaOCl:Tb phosphor having relatively excellent luminosity characteristics, in addition to the mixed phosphor composition of SrGa2S4:Eu phosphor and Y3(Al, Ga)4O12:Tb phosphor, thereby improving the color purity of the Y3(Al, Ga)4O12:Tb phosphor and the degradation characteristics of the SrGa2S4:Eu phosphor.
Based on data shown in Table 1, a conventional mixed green-emitting phosphor containing a Zn2SiO4:Mn phosphor and a mixed green-emitting phosphor composition consisting of a Y3(Al, Ga)4O12:Tb phosphor, a LaOCl:Tb phosphor and a SrGa2S4:Eu phosphor are prepared, and then luminosity characteristics thereof are evaluated under the same conditions as listed in Table 1. The evaluation results are shown in Table 2.
In Table 2, Luminance1) represents a relative luminance when the luminance of the mixed phosphor 1 is assigned to 100%, and the mixed phosphors 1 and 2 are obtained in a composition ratio which is generally known as the best mixture ratio in view of luminosity characteristics including color purity, luminance and degradation characteristics.
Referring to Table 2, the mixed green-emitting phosphor containing SrGa2S4:Eu has improved luminance and degradation characteristics compared to the mixed green-emitting phosphor containing the Zn2SiO4:Mn phosphor. In particular, when the mixed phosphor 2 and the mixed phosphor 3 each containing the Y3(Al, Ga)4O12:Tb phosphor and the LaOCl:Tb phosphor are compared, the mixed phosphor 3 further containing the SrGa2S4:Eu phosphor is better than the mixed phosphor 2 further containing the Zn2SiO4:Mn phosphor, the SrGa2S4:Eu phosphor and the Zn2SiO4:Mn phosphor being contained in the same percent by weight. However, the mixed phosphor 3 has poor color purity compared to the other mixed phosphors, while having an acceptable color coordinate to be used as a green-emitting phosphor. Thus, in order to improve the color purity, it is preferable to increase the content of the SrGa2S4:Eu phosphor within the limit in which the degradation characteristic are not lowered considerably.
Therefore, in the present invention, there is provided a mixed green-emitting phosphor having an excellent luminosity characteristics by replacing a Zn2SiO4:Mn phosphor having poor luminance and degradation characteristics with a SrGa2S4:Eu phosphor having excellent color purity, luminance and degradation characteristics, and mixing a Y3(Al, Ga)4O12:Tb phosphor and a LaOCl:Tb phosphor in a desirable composition ratio.
In the green-emitting phosphor composition consisting of a SrGa2S4:Eu phosphor, a Y3(Al, Ga)4O12:Tb phosphor and a LaOCl:Tb phosphor according to the present invention, the content of the SrGa2S4:Eu phosphor is preferably 5 to 20% by weight, and more preferably 10 to 15% by weight, in view of color purity, luminance and degradation characteristics.
Also, in the green-emitting phosphor composition consisting of a SrGa2S4:Eu phosphor, a Y3(Al, Ga)4O12:Tb phosphor and a LaOCl:Tb phosphor according to the present invention, the content of the Y3(Al, Ga)4O12:Tb phosphor is preferably 60 to 90% by weight, and more preferably 70 to 85% by weight, for improving color purity while maintaining excellent degradation characteristics.
Further, in the green-emitting phosphor composition consisting of a SrGa2S4:Eu phosphor, a Y3(Al, Ga)4O12:Tb phosphor and a LaOCl:Tb phosphor according to the present invention, the content of the LaOCl:Tb phosphor is preferably 5 to 20% by weight, and more preferably 5 to 15% by weight, in view of color purity, luminance and degradation characteristics.
Hereinbelow, the present invention is described more concretely with reference to specific examples intended to illustrate the invention without limiting the scope thereof.